This Core facility generates mutant mice for UCSF Cancer Center investigators. Transgenic mice are produced by microinjecting DNA into the pronuclei of fertilized oocytes or by microinjectiiig preparations of replication-defective lentiviruses into the perivitelline space of fertilized oocytes. The Core performs gene targeting experiments for investigators using mouse embryonic stem (ES) cells. It also generates chimeric mice by blastocyst microinjection of mutant ES cells. Together the transgenic and targeted mutagenesis components of the Core facility allow for the production of a broad range of mutant alleles in the mouse genome for cancer-related research at UCSF. New technologies involving the modification of bacterial artificial chromosomes (BACs) by homologous recombination in E. coli, the use of BAG targeting vectors to generate mutations with high efficiency in ES cells, and the screening for targeted mutations in ES cells by real-time PCR and FISH procedures will be added to the Core facility during the coming funding period. The Core also performs embryo transfers to allow rederivation of imported and/or infected strains of mice into the specific pathogen-free mouse facility, and will be instituting procedures for mouse embryo cryopreservation in the near future.